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There are two modes in which your meter will be most useful - measuring resistance and voltage - so it would be good to learn how to use those. You might be able to find on line a manual for your meter that will help explain these settings. In these modes, it is possible to test different parts of your circuit quickly to find faults and loose connections. (Measurement of current (amps) is less informative in your setting and more difficult to do, so there are settings on your meter that you won't be using to start.)

Measuring resistance (which may require fresh batteries in your meter) will tell you if a circuit is in continuity. Whether the ground really connected to ground, for example, or how well things are connected.

Measuring voltage will tell you if you are delivering 120 volts on the correct lead, or (to take Kurt's example) if you have a voltage drop because of a long wire run or some other loss. Shorts can leak current creating shock and fire hazards.

Any good meter has a continuity setting on it and I am not a fan of using the Ohm setting to check continuity.
Mostly because I can look at what I am doing and hear the beep.

Next yesterday I jumped out the 20 year old young man for using the Ohm setting to check a contact on a live 5 volt circuit with a resistor in series.
He said you could and I said you are getting garbage and learn to use your meter correctly and put it on volts.
No you can, see.
No #$%^ use the thing right, what you are doing is a sure indication of someone that doesn't know how to use a meter.
Please stop arguing and learn.
I did this nicely with my hand on his shoulder, he didn't get mad.
Look 5 volts DC open contact 1.6 volts DC closed contact.
The hard headed little devil.

Not when it comes to instrumentation wires.
The green and white have 5 volts on them going through a resistor that knocks it down to 1.6 volts.
The closed contacts open up when someone moves the volts jump up to around 5 and the system goes into alarm.
You can also use these same motion detector devices to open up or unlock doors and anything else your heart desires.